The present invention relates to methods and systems for liquid-metal magnetohydrodynamic power conversion, especially applicable for use in installations working in the wet-vapor regime, in particular for mixer- and separator-less liquid-metal magnetohydrodynamic power conversion.
The large majority of the concepts of Liquid Metal Magneto-Hydro-Dynamic (LMMHD) Power Conversion Systems (PCSs) proposed so far call for the use of, among other things, a two-component, immiscible fluid system, a mixer and a separator. The two-component fluid consists of a mixture of an electrodynamic fluid--usually a liquid metal, and a thermodynamic fluid--usually a gas (such as helium) or dry vapor (such as dry steam). The thermodynamic fluid expands nearly isothermally (due to its being in direct contact with the liquid metal, which has a relatively large heat capacity) into a lower pressure regime, carrying along with it the electrodynamic fluid. If the thermodynamic fluid is of the volatile type, expansion takes place in the superheated vapor regime. These processes lead, to the conversion of thermal energy into electricity in a two-phase MHD generator. After expansion in the MHD generator, the thermodynamic fluid is separated (in the separator) from the electrodynamic fluid (in several concepts, separation takes place upstream of the MHD generator), is cooled, compressed back to the high pressure of the cycle, heated back to the high temperature of the cycle, mixed (in the mixer) with the electrodynamic fluid, thus completing the cycle.
One drawback of the LMMHD PCSs based on the use of the conventional two immiscible component fluid system is the need for a mixer and, particularly, of a separator. The latter component is not only liable to lead to a significant loss in efficiency, but is complicated to design particularly for space (i.e., zero gravity) applications. Another drawback of these LMMHD PCSs is that they cannot match well a power system which provides its energy over a wide temperature range; the near isothermal expansion can only take place at a temperature significantly below the upper temperature of the heat source, thus limiting cycle efficiency.
A single-component LMMHD PCS is known which requires no mixer or separator. This system uses a "condensing-injector" in which the condensation of the vapor is achieved by injecting a sub-cooled liquid into the two-phase mixture prior to entering the MHD generator. In the condensing injector, the vapor is condensed and a high-stagnation head liquid is generated. The liquid passes through a single-phase MHD generator, where electric energy is generated at the expense of the stagnation pressure head. The performance of this cycle is determined essentially by the performance of the condensing injector. Unfortunately, several studies have shown that this device has an inherently low internal efficiency due, primarily, to the fact that the streams of vapor and liquid enter it at significantly different velocities; the encounter of two such streams is associated with a significant loss of kinetic energy. After a number of theoretical and experimental studies carried out both in the United States and in the USSR indicated that the efficiency of the condensing injector and, therefore, of the cycle based on this device, was too low to be of practical interest, all further research on condensing-injector-based PCSs was abandoned in the late sixties.
In none of the other PCSs designed to have a jet condenser was there an attempt to match the velocities of the subcooled liquid and the vapor or two-phase stream.
Another prior-art system proposes using a two-component, separator-less LMMHD PCS. The two-component fluid consists of a liquid metal for the electrodynamic fluid, and an organic liquid for the thermodynamic fluid. Upon heating, the organic liquid vaporizes to form a two-phase, high-pressure mixture. The organic fluid vapors expand to the low pressure of the cycle, carrying with them the liquid metal through the MHD generator to produce electricity. At the low pressure part of the cycle the mixture is cooled, thus causing the organic fluid vapor to condense and form a single liquid phase. After being pumped to the high pressure of the cycle, the mixture is heated up to completely vaporize the organic fluid to provide the desirable void fraction. Subsequent expansion of the fluid takes place in the superheated vapor regime. As the liquid metal and organic fluids are immiscible, special mixers need be installed in the system to ensure a homogeneous mixture.
A common limitation of the LMMHD PCSs which use a two-phase MHD generator is that the maximum gas or vapor volume fraction in the cycle is less than about 85%; at higher void fractions conductivity of the two-phase mixture starts to drop, thus impairing the efficiency of the MHD generator. Had there been no loss in conductivity, the higher the cycle maximum void fraction, the higher could be the expansion pressure ratio and the efficiency attainable from a given stage of LMMHD PCS.
Two approaches enabling to increase the maximum void fraction attainable in the expansion of the two-phase mixture in LMMHD PCSs have been proposed. One approach is to add to the working fluid so-called surfactants, i.e., surface-tension reducing additives. These additives promote foaming which enables expanding the two-phase mixture in the MHD generator channel to a much higher exit void fraction than otherwise possible without loss in MHD generator efficiency. As foaming interfers with the separation of the gaseous phase from the liquid phase, it is only practical in LMMHD PCSs which are separator-less.
Another approach proposed to obtain higher void fractions than attainable in MHD generators is to allow the working fluid to continue the expansion process in a nozzle installed downstream of the generator. The extra conversion of thermal-to-kinetic energy thus achieved can be turned into a pressure head in a diffuser to follow the nozzle. To make this process efficient, though, it is necessary to separate the vapor from the two-phase mixture exiting the nozzle, to enable the use of a liquid diffuser. However, the separation process in itself introduces losses in efficiency and complicates design.